Silica refractory shapes



Aug. 11, 1964 D. o. M CREIGHT ETAL 3,144,345

SILICA REFRACTORY SHAPES Filed May 5, 1961 MF'LE. 1

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United States Patent 3,144,345 SILHCA REFRACTORY SHAPES Donald 0. Mctlreight, Bethel Park, and Ben Davies and Albert L. Renkey, Pittsburgh, Pa., assignors to Harbisou-Walker Refractories Company, Pittsburgh, Pa., a corporation of Pennsylvania Filed May 5, 1961, Ser. No. 107,932 4 Claims. (Cl. 106-58) This invention relates to improved silica refractory shapes and brick for use in refractory service, for example as wall lining materials in by-product coke ovens and the like.

By-product coke ovens are long narrow chambers lined with silica brick and usually joined together in batteries of up to 100 or more ovens. The ovens are separated from each other by silica brick walls which also enclose the heating fiues that supply the heat for coking the coal. By this means, the combustion gases from the heating fines and the gaseous products of carbonization are kept separate at all times.

In the operation of coke ovens, the ovens are readied for chmging by setting the doors in place. The coal is then charged into the coking chambers and the firing cycle begun. Coking is normally completed in 14 to 18 hours depending on the width of the oven and the temperatures employed. When the coking cycle is completed, the doors are removed and the coke is pushed out of the oven by an electrically driven ram into the quenching car. The quenching car transfers the hot coke to the quenching station where the coke is cooled by spraying with water. the oven, the doors are replaced and the oven prepared for charging again.

Silica brick have been the standard refractory used in the construction of coke ovens for many years. Their ability to withstand abrasion and high loads at elevated temperatures and their volume stability at high temperatures have made them particularly suitable for this application.

Silica brick are well known in the art for their load bearing capacity up to within a few degrees of their softening point. This quality is important in coke oven construction where high stresses are encountered at operating temperatures. Conventional coke oven silica brick Withstand a load of 25 p.s.i. up to about 3000 F.

The thermal expansion of silica brick is essentially complete when a temperature of about 1060 F. is reached. This feature obviates the necessity to provide for reversible expansion at higher temperatures. Thus, silica brick undergo no reversible change when heated and cooled at the operating temperatures of the coke oven, 2000 F. to 2800 F.

In the construction of coke oven batteries, the ovens are separated from each other by silica brick walls which also enclose the heating flues that supply the heat for coking the coal. It can be seen that the coking rate is dependent upon the rate of heat transfer through the brick from the flues to the coal. It has been learned that silica brick of high density have greater thermal conductivity at coke oven operating temperatures than those of lower density. Therefore, this invention is directed toward increasing the density of silica brick, there by providing a brick of greater thermal conductivity. Ancillary benefits also received when the density of silica brick is increased include: greater strength, which is directly related to abrasion resistance of a given material, and increased resistance to penetration and attack by fumes and gases due to a decrease in the porosity of the After the coke is pushed out of ice brick. Abrasion resistance is an important factor in coke oven life due to the impact of the coal being charged and the wear resulting from the coke being pushed out of the oven after the coking is completed.

It is, therefore, a major object of the present invention to provide silica shapes such as brick that are characterized by higher density, increased resistance to abrasion and higher thermal conductivity than presently experienced in silica brick of commerce.

It is another object of the present invention to provide improved articles such as coke oven walls and the like constructed of such brick having all the advantages set forth in the preceding object and further advantageous in that the surfaces therein are unusually smooth.

The attached drawing shows thermal conductivity data on silica refractory products in accordance with this invention.

These and other objects are attained in accordance with our invention in which about 1 to 5 weight percent of finely divided volatilized, amorphous, non-vitreous silica, based on the solids content of the resulting batch, is included in a silica refractory batch. Silica brick produced therefrom, and otherwise made in accordance with standard refractory practices are characterized by increased resistance to abrasion, higher density, smoother surfaces and higher thermal conductivity than are silica refractories produced heretofore, all of the advantages being obtained without impairing the refractoriness of the product.

The silica refractories with which the invention is concerned are formed from a batch composed, by weight, of 1 to 5 percent total of at least one member of the group consisting of calcium oxide and magnesium oxide, 1 to 5 percent of volatilized, non-vitreous silica, and the remainder silica rock or quartzite. For superduty brick, chemical analysis of the batch will show not over 0.5 percent total of alumina (A1 0 titania (TiO and alkalies (Na O+K O). For conventional silica brick, those materials may range in the aggregate up to about 0.8 to 1.5 percent. The composition is further characterized in the instance of superduty quality silica brick, in that chemical analysis will show that one or more members of the group consisting of calcium oxide and magnesium oxide are present in a total amount of at least 3.3 times the content of alumina, titania and alkalies.

The line (C210) and magnesia contents are supplied by that added as bond, usually in the form of commercial hydrates. In one particular embodiment, markedly improved surface quality is obtained in either regular or superduty quality brick.

The finely divided amorphous silica used in manufacture of refractories in accordance with this invention is fumed or volatilized silica,'that is silica which has been deposited from a vapor phase. It is used in amounts up to about 5 Weight percent of the batch solids, and preferably 2 to 5 percent. Such amorphous silica is a byproduct of the reduction of silica to form silicon alloys, such as ferrosilicon. Silica fume has also been produced by reducing quartzite with carbon, treating the vaporous products of reduction with air or other oxygen yielding gas, and condensing the silica in finely divided form. Amorphous silica as thus prepared or made from any other vaporizing process can be used in this invention. As used, it must be substantially all finer than 50 microns and at least one-half of it finer than one micron in size. The amorphous silica normally analyzes about percent SiO with about 2 to 3 percent of FeO+MgO+Al O and about 2 percent ignition loss. A further characteristic of this amorphous silica is that it is in the form of very fine spherical particles. These particles are readily distinguished from the angular particles resulting from grinding or the angular, platelike or fibrous particles formed by natural deposition, regardless of the fineness of the microstructure. Of course, other silica is generally crystalline in structure whereas that used in this invention is amorphous.

The reason for the densifying action of amorphous fume silica as an ingredient in silica brick as in this invention is not clearly understood. In the past, silica fume has been used as an additive to basic refractory materials where its ability to act as a bond was found to be the result of a reaction between the silica fume and the basic material. However, it is obvious that such cannot be the case in our invention where the aggregate is silica. Therefore, the effects we obtain cannot be the result of any chemical reaction.

Likewise, the very fineness of the particles does not account for the improvements in properties we obtain. We have attempted to obtain the same results by replacing the amorphous silica with crystalline silica of any other origin and fineness, but without success. We have theorized that surface electrical eifects on the particles are involved. The fume, having been generated by the oxidation of silicon vapor, may have all molecular bonds within its particles satisfied, while fine particles of silica produced by any fragmentation method would have broken molecular bonds at the surface and, hence, there could be present repulsive electrical forces.

The silica rock or quartzite used in the compositions of the invention can be any of the varieties commonly used in making silica brick. As mined, the silica material may consist of quartzite in massive form or as agglomerated quartzite pebbles. Other forms of silica regularly used for silica brick manufacture are also suitable.

The lime used for bonding will ordinarily be commercial hydrated lime. Dolomitic lime (CaO.MgO) is also usable and will likewise ordinarily be used as the hydrate. When magnesia (MgO) is used alone, it will be preferable to use the lightly burned magnesia (caustic magnesia) which is readily hydratable. There is nothing in these practices which is not well known in the art of silica brick manufacture. The lime or magnesia added to the batch in these forms is spoken of as the bond, since it is available both as a bond in the fired brick and also for giving strength to the unfired brick. In silica brick manufacture, lime is commonly used in amounts of 1 to 5 percent( on the basis of CaO), and magnesia which has similar properties is less commonly used.

Silica brick of the invention are usually made by the power press, impact press, or hand molding process in accordance with standard techniques developed in the production of silica refractories. In the following examples, the standard power press method of making silica rick was employed. The components were crushed and thoroughly blended together to give a typical brickmaking grind, as follows:

Percent 6+10Tyler mesh 6 10+28 36 28+65 16 -65 42 About 5 percent by weight of water was added as was about one percent of concentrated Waste sulfite liquor, a temporary bonding agent. The batch was then pressed into brick, measuring 9 x 4 /2 x 3 inches, at about 4000 p.s.i. The brick were removed from the press and dried for about 24 hours at 250 F. The brick were then fired in a tunnel kiln for five days, reaching a top temperature of 2700 F.

In these examples, a conventional coke oven silica brick batch of silica rock or quartzite and lime (CaO), added as hydrated lime, as bonding agent, was varied by substituting minor amounts of volatilized silica for the silica aggregate. The silica aggregate used in this examples was a 4. Pennsylvania quartzite having the following chemical analysis.

Percent SiO 99.57 A1 0 0.16 Fe O 0.21 T10 0.03 Alkalies 0.03

The batch components and data obtained on the resulting brick are:

Table I Pennsylvania Quartzite "percent" 95.7 94. 7 92. 7 90. 7 88.7 Lime Hydrate do-- 3. 3 3. 3 3. 3 3. 3 3. 3 volatilized Silica d 0.0 1.0 3. 0 5. 0 7. 0 Waste Sulfite Liquor. 1.0 1.0 l. 0 1. 0 1. 0 Bulk Density, 1b./tt 3 112 114 115 113 Porosity 3. 7 22. 5 21. 3 21. 0 21. 4 Modulus of Runtur 770 830 860 850 830 Permeability, infi/see/infi 0t area/in.

thickness/p.s.i. pressure 0.5 0. 4 0.2 Abrasion Loss, loss in volume in ec.. 2. 86 2. 75 2. 42

It can be observed that the addition of silica fume improved each of the properties tested. An almost 5 percent increase in bulk density was obtained over the standard mix (No. 1) when 5 percent of the volatilized silica was added (Example 4). Likewise, about a 10 percent decrease in porosity resulted from the 5 percent addition of silica fume. A 10 percent increase in strength, as denoted by modulus of rupture, is also apparent. The .beneficial effect on resistance to abrasion is also evident in the 15 percent decrease in abrasion loss with a 5 percent addition of volatilized silica. The increase in thermal conductivity resulting from the addition of volatilized silica is demonstrated in the curves in the attached draw- The addition of 5 percent volatilized silica gave the brick about a 5 percent increase in thermal conductivity.

As can be seen from the foregoing, the addition of volatilized silica provides silica brick having increased density, increased strength, lower porosity, lower permaability, increased abrasion resistance, and higher thermal conductivity. It will be observed in Table I that the addition of 5 percent fume silica gives the best results. Alditions of more than 5 percent, though productive of brick more suitable for coke oven use than those presently commercially available, are not warranted because the most favorable effect on the properties has been passed. Thus, the 7 percent addition described in Example 5 displays approximately the same improvement as the 3 percent addition of Example 3.

The foregoing brick mixes show the addition of amorphous, volatilized silica to the conventional silica brick mix of graded silica particles and lime. Volatilized silica also is effective for the stated purposes with small amounts of other additions, such as alumina and clay, which can be added in amounts of a few percent.

The volatilized silica as an additon to the silica brick mix has been found to result in another unexpected advantage relating to the nature of the finished surfaces. In certain service applications for silica brick, for example the coke oven, it has been considered necessary to have a very smooth Wearing surface. It has been felt that the coke will abrade the surface less rapidly if its is well filled out and free of roughness. All coke oven liner brick are minutely inspected before installation, and many otherwise satisfactory brick are rejected because of the quality of the surface. This is a very sizeable economic waste, for which many have sought a remedy. Surprisingly, we have found that our addition of a few percent of volatilized silica is a most effective corrective for surface roughness.

To obtain the very greatest improvement in surface improvement, we prefer to form our silica brick on the impact or vibratory presses which are employed extensively in silica brick manufacture. Such presses are particularly well suited to the forming of the intricate shapes required in coke oven construction. The impact press produces shapes of high density and low porosity.

The fact that volatilized silica provides silica brick with markedly smoother surfaces has had another important result related to the removal of batching limitations. In many of the applications of silica brick, resistance to thermal shock is a limiting factor, because the furnace temperatures fluctuate in the process of operating the furnace. In the refractories art, a traditional means of improving the property of thermal shock resistance has been to coarsen the grind, by using either coarser particles in the mix, or a greater percentage of the larger particles already present in the gradation of sizes which comprises the brick batch. In the past it was impossible to accomplish very much by these means for silica brick in general, and for coke oven brick in particular, because the coarser particles impaired the surfaces, and weakened the edges and corners of the brick prohibitively. These physical characteristics are so greatly improved by our discovery, that the refractories manufacturer is given an important new latitude in controlling the thermal shock resistance by coarsening the grind, while utilizing the improvements resulting from the presence of 1 to 5 percent volatilized silica.

Our invention provides brick which permit coke ovens to be operated more efliciently due to the increase in thermal conductivity and insure a longer service life due to the increase in density, strength, and abrasion resistance. One of the most significant factors to be observed is that these improvements were imparted to the brick through the addition of a substance which does not deleteriously affect the refractoriness of the final product. This factor is of great importance to the user of refractories because of the modern trend of operating at higher and higher temperatures to expedite production.

Our invention has been related particularly to coke oven silica brick, because the immediate utility of the new products is economically so important in that application. However, it should be appreciated that the improved refractory products can be used in any other application desired, and the advantages stemming from the invention will be important in many other furnace applications.

In accordance with the provisions of the patent statutes, we have explained the principle of our invention and have described what we now consider to represent its best embodiment. However, we desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

We claim:

1. As a smooth-surfaced wall lining for a coke oven, a plurality of smooth-surfaced fired brick made from a brickmaking size graded batch consisting essentially of, by weight: about 1 to 5 percent total, on the basis of an oxide analysis, of at least one member selected from the group consisting of lime hydrate, lightly calcined magnesium oxide and mixtures thereof, as a fired bonding agent; about 1 to 5 percent, based on the total batch, of a temporary bonding agent which is capable of being burned out on firing; about 1 to 5 percent of generally spherical particles of fumed silica in order to obtain said smooth surfaced brick on firing, substantially all the particles of the fumed silica being smaller than about 50 microns, and the remainder of the batch being crushed, size graded, silica rock.

2. A wall lining in accordance with claim 1, said fumed silica being present in an amount of about 2 to 5 percent, by weight of the batch.

3. A smooth-surfaced fired brick made from a brickmaking size graded batch consisting essentially of, by weight: about 1 to 5 percent total, on the basis of an oxide analysis, of at least one member selected from the group consisting of lime hydrate, lightly calcined magnesium oxide and mixtures thereof, as a fired bonding agent; about 1 to 5 percent, based on the total batch of a temporary bonding agent which is capable of being burned out on firing; about 1 to 5 percent of generally spherical particles of fumed silica in order to obtain said smooth surfaced brick on firing, substantially all the particles of the fumed silica being smaller than about 50 microns, and the remainder of the batch being crushed, size graded, silica rock.

4. The method of producing silica brick having outstandingly smooth surfaces comprising the steps of, forming a refractory brickmaking size graded batch having a composition consisting essentially of, by weight, about 1 to 5 percent, on the basis of an oxide analysis, of at least one member selected from the group consisting of lime hydrate, lightly calcined magnesium oxide and mixtures thereof, about 1 to 5 percent of generally spherical particles of fumed silica substantially all the particles of which are smaller than about 50 microns in size, and the remainder being crushed, size graded, silica rock, tempering said batch by adding about 1 to 5 percent, based on the batch, of an aqueous temporary bonding agent capable of being burned out on firing, pressing brick from the resulting tempered batch, drying the resulting brick, and firing the dried brick to about 2700 F. to obtain smooth-surfaced silica brick.

References Cited in the file of this patent UNITED STATES PATENTS 2,351,204 Harvey et a1. June 13, 1944 FOREIGN PATENTS 467,555 Canada Aug. '22, 1950 

1. AS A SMOOTH-SURFACED WALL LINING FOR A COKE OVEN, A PLURALITY OF SMOOTH-SURFACED FIRED BRICK MADE FROM A BRICKMAKING SIZE GRADED BATCH CONSISTING ESSENTIALLY OF, BY WEIGHT; ABOUT 1 TO 5 PERCENT TOTAL, ON THE BASIS OF AN OXIDE ANALYSIS, OF AT LEAST ONE MEMBER SELECTED FROM THE GROUP CONSISTING OF LIME HYDRATE, LIGHTLY CALCNED MAGNESIUM OXIDE AND MIXTURES THEREOF, AS A FIRED BONDING AGENT; ABOUT 1 TO 5 PERCENT, BASED ON THE TOTAL BATCH, OF A TEMPORARY BONDING AGENT WHICH IS CAPABLE OF BEING BURNED OUT ON FIRING; ABOUT 1 TO 5 PERCENT OF GENRALLY SPHERICAL PARTICLES OF FUMED SILICA IN ORDER TO OBTAIN SAID SMOOTH SURFACED BRICK ON FIRING, SUBSTANTIALLY ALL THE PARTICLES OF THE FUMED SILICA BEING SMALLER THAN ABOUT 50 MICRONS, AND THE REMAINDER OF THE BATCH BEING CRUSHED, SIZE GRADED, SILICA ROCK. 